1.1. Field of the Invention
This invention relates to the field of well logging, and more particularly, to improved shields for use with logging instruments using sources or sensors having a transverse or tilted magnetic dipole.
2.2. Description of Related Art
Electromagnetic (EM) logging instruments have been employed in the field of hydrocarbon exploration and production for many years. These logging instruments or xe2x80x9csondesxe2x80x9d entail an elongated support member equipped with antennas that are operable as sources and/or sensors. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support. U.S. Pat. No. 4,873,488 (assigned to the present assignee), for example, describes logging instruments equipped with antennas disposed along a central support.
In operation, a transmitter antenna is energized by an alternating current to emit EM energy into the formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more antennas. The measured signals are then processed to determine the electrical properties, such as permittivity or conductivity, of the formation.
Conventional EM logging techniques include xe2x80x9cwirelinexe2x80x9d logging and logging-while-drilling (LWD). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations. A new method sometimes referred to as logging-while-tripping (LWT) involves placing a logging tool near the bottom of the drill string and making measurements while the string is withdrawn from the borehole.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the product of the current and the area encompassed by the coil. The direction and strength of the magnetic moment can be represented by a vector perpendicular to the plane of the coil. In the case of more complicated coils which do not lie in a single plane (i.e. saddle coils referenced later), the direction of the dipole moment is given by: o∫rxc3x97dl and is perpendicular to the effective area of the coil. This integral relates to the standard definition of a magnetic dipole of a circuit. See J. A. Stratton, ELECTROMAGNETIC THEORY, McGraw Hill, New York, 1941, p. 235, FIG. 41. Integration is over the contour that defines the coil, r is the position vector and dl is the differential segment of the contour.
In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along, or parallel, to the longitudinal axis of the instrument. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD).
If the transmitter and receiver antennas on these instruments were perfectly configured and balanced in a theoretically ideal system, the EM energy emitted by the antennas would propagate in a mode known as a transverse electric (TE) mode, of the type generated by an ideal vertical magnetic dipole in an azimuthally symmetric media. However, under actual operating conditions, there are various factors that give rise to the generation of significant undesired EM field components. One approach to alleviating this problem is with the use of antenna shields to reduce the transmission and/or reception of spurious and unwanted EM field components. These shields are typically used in conjunction with each antenna on the instrument although they can be used with only some of the antennas. For instance, if each shield provides N dB attenuation of undesired modes, then having shields on both transmitters and receivers will provide 2N dB of attenuation. If N dB is enough for the measurements desired, then shields may be used only for the transmitters or only for the receivers.
U.S. Pat. Nos. 5,631,563, 4,808,929, 4,949,045, and 4,536,714 (all assigned to the present assignee) disclose conventional antenna shields used with these instruments to provide mechanical protection for the antennas and to permit the passage of desired EM field components. As shown in FIG. 1a, these shields 10 are in the form of a metal cylinder that has slots 12 in the axial direction. The slot 12 pattern allows the azimuthal electric field (Excfx86) component to pass through with little attenuation, while the radial (Er) and axial (Ez) are attenuated more as they pass through the shield.
An alternative viewpoint is to represent each axial slot 12 as an axial magnetic dipole, as shown in FIG. 1b. These magnetic dipoles couple to axial magnetic fields (Bz), but do not couple to azimuthal magnetic (Bxcfx86) fields. The shielded antennas are thus rendered substantially insensitive to parasitic transverse magnetic (TM) EM fields associated with borehole modes, and which have radial (Er) and axial (Ez) electric fields and azimuthal magnetic fields (Bxcfx86).
An emerging technique in the field of well logging is the use of instruments with tilted or transverse antennas, i.e., where the coil""s axis is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole moment (TMD). One instrument configuration comprises tri-axial coils, involving three coils with magnetic moments that are not co-planar. The aim of these TMD configurations is to provide EM measurements with directed sensitivity and sensitivity to the anisotropic resistivity properties of the formation.
Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and 6,147,496. Common to these apparatus and techniques, however, is the need to manipulate the antenna itself. None of these disclosures address the implementation of shields as alternative means to achieve selective EM energy attenuation.
A transverse slot concept has been used in design of high frequency communication antennas. See Shumpert, J. D., and Butler, C. M., xe2x80x98Penetration through slots in conducting cylindersxe2x80x94Part 1: TE case, xe2x80x98IEEE Trans. Antennas and Propagation, vol. 46, pp. 1612-1621, 1998; Shumpert, J. D., and Butler, C. M., xe2x80x98Penetration through slots in conducting cylindersxe2x80x94Part 2: TM case, xe2x80x98IEEE Trans. Antennas and Propagation, vol. 46, pp. 1622-1628, 1998; Park, J. K., and Eom, H. J., xe2x80x98Radiation from multiple circumferential slots on a conducting circular cylinder, xe2x80x98IEEE Trans. Antennas and Propagation, vol. 47, pp. 287-292, 1999. These papers present methods for modeling the EM field. However, the concept and physical setup in communications applications is different from that involved with logging applications. A key difference being the frequency range of operation: logging instrument antennas generally operate in EM diffusion regime while communication antennas operate in propagation regime, where dimensions of antennas and slots are comparable to wavelength.
It is desired to implement a technique to produce a pure transverse magnetic dipole EM field for subsurface formation measurements. Still further, it is desired to implement a shield apparatus that can be used in conjunction with saddle, tilted coils or multi-layered tri-axial coils to produce such a field.
A shield apparatus adapted for use in conjunction with a logging instrument provides predetermined attenuation of one or more electromagnetic energy field components as the field interacts with the shield.
One aspect of the invention is an apparatus for use with an elongated support that is adapted for disposal within a borehole, the support having a longitudinal axis. The apparatus comprises a body adapted to form a cylindrical surface; the body being adapted for mounting on the support; and the body having at least one slot formed therein such that the slot is perpendicular to the longitudinal axis when the body is mounted on the support; wherein the body provides predetermined attenuation of an electromagnetic field component as the field interacts with the body.
Another aspect of the invention is an apparatus for use with an elongated support that is adapted for disposal within a borehole, the support having a longitudinal axis. The apparatus comprises a flexible strip adapted to surround the support, the strip being formed of a non-conductive material; and at least one conductive element disposed on the strip such that the element is perpendicular to the longitudinal axis when the strip surrounds the support; wherein the strip provides predetermined attenuation of an electromagnetic field component as the field interacts with the strip.
Another aspect of the invention is a system for measuring a property of a subsurface formation. The system comprises an elongated support having a longitudinal axis, the support being adapted for disposal within a subsurface borehole traversing the formation; a source or sensor is mounted on the support; a shield is mounted on the support to cover the source or sensor; and the shield has at least one slot formed therein, the slot being perpendicular to the longitudinal axis of the support; wherein the shield provides predetermined attenuation of an electromagnetic field component as the field interacts with the shield.
Another aspect of the invention is a system for measuring a property of a subsurface formation. The system comprises an elongated support having a longitudinal axis, the support being adapted for disposal within a subsurface borehole traversing the formation; a source or sensor is mounted on the support; a flexible strip is mounted on the support to cover the source or sensor; and the strip has at least one conductive element disposed therein, the element being perpendicular to the longitudinal axis of the support; wherein the strip provides predetermined attenuation of an electromagnetic field component as the field interacts with the strip.
Another aspect of the invention is a method for shielding a source or sensor disposed on an elongated support having a longitudinal axis and adapted for disposal within a borehole. The method comprises mounting a body adapted to form a cylindrical surface on the support to cover the source or sensor, the body having at least one slot formed therein such that the slot is perpendicular to the longitudinal axis, wherein the body provides predetermined attenuation of an electromagnetic field component as the field interacts with the body.
Another aspect of the invention is a method for shielding a source or sensor disposed on an elongated support having a longitudinal axis and adapted for disposal within a borehole. The method comprises mounting a flexible strip on the support to cover the source or sensor, the strip having at least one conductive element disposed therein such that the element is perpendicular to the longitudinal axis, wherein the strip provides predetermined attenuation of an electromagnetic field component as the field interacts with the strip